The present invention relates generally to data frame synchronization for use in telecommunications systems and more specifically for airframe synchronization through use of a Discrete Phase Locked Loop Solution.
Telecommunications systems include various elements within the system that need to be synchronized, to allow data communication between the system's elements. In order to provide system synchronization, a communications system should distribute accurate frequency and time reference signals. For example, in a time division multiple access (TDMA) mobile communications network, a base station transmits bursts of data known as airframes (or simply frames), to mobile units traveling in an area serviced by the base station. In an American Digital Cellular (ADC) system for example, a frame is defined as a digital packet containing six time slots transmitted at a 25 Hertz frame rate. As illustrated in FIG. 1, this exemplary frame format is used in the D-AMPS system specified in EIA/TIA IS-54B. However, those skilled in the art will appreciate that other systems, such as those specified by Global System For Mobile Communication (GSM), may provide different frame/time slot formats and timing.
Consider the situation depicted in FIG. 2. An original base station BS1 is handling a connection between mobile station MS and the network as represented by the transmission link TL1 between base station BS1 and the mobile switching center MSC. The mobile station MS then moves to a position MS' where it is then determined that this connection would best be handled by base station BS2, e.g., to improve the signal quality of the connection. The system initiates a handoff procedure by sending appropriate commands to base stations BS1 and BS2 over transmission links TL1 and TL2. The mobile station MS may or may not be informed of the impending handoff.
At some time after the handoff decision is made, transmissions will begin from the base station BS2 and terminate from base station BS1. In some cases, e.g., where a mobile station has the capability of performing diversity combination or selection of plural signals, it may be desirable to allow transmission to continue from both base stations for some time period. In other cases, it may be desirable to have little or no overlap in the transmissions from base stations BS1 and BS2. In either scenario, it is important to ensure that no frames are lost during the handoff procedure. Thus, it is desirable that the mobile station cleanly receive a last frame from the original base station BS1 followed by a first frame from base station BS2. This involves at least two timing aspects: (1) estimating the difference in propagation delay between the original base station BS1 and the mobile station MS, and that between the new base station BS2 and the mobile; and (2) synchronizing the transmissions between the base stations so that the frames from each base station arrive at the mobile station at the desired times.
However, providing such synchronization is difficult as there is very little gap time between the transmitted frames. In order to synchronize the transmission of the frames of the two different base stations, BS1 and BS2, a highly accurate and quickly discernible reference signal is needed such that the base stations are time synchronized within, for example, 2 microseconds to ensure the frame decoder in the mobile will not be disturbed by lost or duplicated data.
A second application for the synchronization of airframes in telecommunications systems occurs when a single base station contains multiple transceivers that are each transmitting the same, or substantially the same, information to a mobile unit. The transceivers can be separated within the same base station or base station site or transceivers from neighboring sites can cooperate for a call handled by a common switching center, wherein the neighboring sites are globally synchronized. Each transceiver can transmit at slightly different frequencies in order to avoid interference. As the base station transmits the airframes to a mobile unit, the mobile unit receives each of the signals and combines them such that the signals appear much stronger. This is often referred to as simulcasting. Simulcasting may be achieved by synchronizing the airframe timing of two transceivers and having the transceivers transmit with a known offset relative to each other. However, in order for the mobile station to be able to combine the signals, the transmission of the signals by the base stations must be synchronized. For this application, synchronization between base station transmitters should be determined within, for example, ten microseconds.
Airframe synchronization has not previously been implemented. In order to synchronize the airframes, both airframe data clocks and synchronization signals must be phase locked to a global time reference signal. To minimize system down time, it is desirable to lock synchronization to the reference signal quickly. However, by attempting to reach a locked condition quickly, the chance of vector error is increased which in turn could compromise communications from the transceiver. Therefore, it is desirable to have a communications system that provides a locked condition as quickly as possible without losing an unacceptable amount of data or the connection to the transceiver.
One possible method for providing synchronization would be to use a conventional analog phase-locked loop (PLL). An analog PLL typically contains a voltage controlled crystal oscillator (VCO), a phase comparator, and a low pass filter. The VCO is controlled by the voltage from a low pass filter derived from a phase comparator. The phase comparator compares an incoming reference frequency with a frequency generated by the VCO. In order to provide the accuracy needed to establish synchronization for the applications described above using a conventional analog solution, a VCO with performance better than one part per million (PPM) frequency deviation should be used. However, this type of VCO is very expensive, and its implementation in a PLL is also space demanding.
Another drawback of an analog PLL is the amount of time required to achieve a locked condition including inherent delays that cannot be overcome as the VCO itself will be the source of certain failures and inaccuracies. These delays would make implementation of global airframe synchronization impractical using an analog PLL. For example, to obtain a locked condition using a conventional analog PLL would take on the order of 30 to 70 minutes without compromising a call. In the case of a loss of system power or system soft-reboot, it could take up to 70 minutes to achieve synchronization of the transceivers using an analog PLL. This length of time would be unacceptable for handling calls between a base station and a mobile phone. Moreover, as with all analog devices, an analog PLL is subject to additional inaccuracies attributable to aging of its components.
It is therefore an object of this invention to provide fast dynamic synchronization through use of a global time reference signal without many of the above-described drawbacks. It is also an object of the invention to provide a phase-locked loop having a reduced manufacturing cost that is significantly smaller than conventional analog PLLs. It is a further object of the invention to provide a PLL that suppresses reference error noise, is not adversely affected by aging, and enables faster phase lock response times.